We present results from light scattering experiments on tetragonal FeS with the focus placed on lattice dynamics. We identify the Raman active A1g and B1g phonon modes, a second order scattering process involving two acoustic phonons, and contributions from potentially defect-induced scattering. The temperature dependence between 300 and 20 K of all observed phonon energies is governed by the lattice contraction. Below 20 K the phonon energies increase by 0.5-1 cm −1 thus indicating putative short range magnetic order. Along with the experiments we performed latticedynamical simulations and a symmetry analysis for the phonons and potential overtones and find good agreement with the experiments. In particular, we argue that the two-phonon excitation observed in a gap between the optical branches becomes observable due to significant electronphonon interaction.

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Combined resistance noise and muon-spin relaxation (µSR) measurements of the ferromagnetic semiconductor HgCr2Se4 suggest a degree of magnetoelectric coupling and provide evidence for the existence of isolated magnetic polarons. These form at elevated temperatures and undergo a percolation transition with a drastic enhancement of the low-frequency 1/f -type charge fluctuations at the insulator-to-metal transition at ∼ 95 − 98 K in the vicinity of the magnetic ordering temperature TC ∼ 105 − 107 K. Upon approaching the percolation threshold from above, the strikingly unusual dynamics of a distinct two-level fluctuator superimposed on the 1/f noise can be described by a slowing down of the dynamics of a nanoscale magnetic cluster, a magnetic polaron, when taking into account an effective radius of the polaron depending on the spin correlation length. Coinciding temperature scales found in µSR and noise measurements suggest changes in the magnetic dynamics over a wide range of frequencies and are consistent with the existence of large polarized and domain-wall-like regions at low temperatures, that result from the freezing of spin dynamics at the magnetic polaron percolation transition.